Semiquantum Key Distribution Using Entangled States

Recently, Boyer et al. presented a novel semiquantum key distribution protocol Phys. Rev. Lett. 99 (2007) 140501 by using four quantum states, each of which is randomly prepared in the Z or X basis. Here we present a semiquantum key distribution protocol by using maximally entangled states in which quantum Alice shares a secret key with classical Bob. Quantum Alice has the ability to prepare Bell states and perform Bell basis or computational basis measurement. Classical Bob is restricted to measuring, preparing a particle in the computational basis, reflecting or reordering the particles. The qubit efficiency of the protocol improves to 50% and the protocol can be modified to a measure-resend protocol or a protocol without quantum memory. We also show that the protocol is secure against eavesdropping.